1. Field of the Present Invention
The present invention relates to a polyester resin containing tungsten-titanium carbide particles, more particularly, to a polyester preform made from the polyester resin is of high clarity and low haze while being more capable of absorbing near infrared, thereby facilitate saving energy consumed in a subsequent bottle blowing process.
2. Description of Related Art
Polyesters, particularly polyethylene terephthalate (also known as PET), are usually used to make plastic bottles for packing beverages and foods for the reason that such PET polyester bottles have outstanding strength, transparency and chemical resistance.
A process for making a polyester bottle is as described below. First, a polyester resin as the material is made through polycondensation reaction. The polyester resin is then melt and injected into preform mold cavities, as the known injection molding procedure, and thus forms a preform after cooling. Afterward, the preform is heated by infrared lamps to a temperature higher than the polyester's glass transition temperature, which is about 20-40□, before sent to the blowing mold and blown into a bottle through a conventional mold blowing procedure.
Currently, preforms are industrially processed through infrared heating, which is typically enabled by quartz infrared lamps, after which the softened PET preforms are placed in molds for a stretch blow moulding process where they are formed into bottles.
The reason for using quartz infrared lamps as the heating source for PET preforms is that infrared rays go through PET preform in a radiative manner, so that PET molecules can be evenly heated by cyclotron resonance, leading to temperature harmonization between the inside and outside of the entire preforms. Otherwise, an unevenly heated PET preform can generate crystals that cause haze during the stretch blow moulding process.
While a quartz infrared lamp typically has a maximum wavelength of 1100-1200 nm, the PET resin under structural resonance can only absorb energy of 5500-10000 nm, which is in the far-infrared region and much different from the near-infrared region in which the radiation emitted by the quartz infrared lamp ranges.
Since near infrared as energy produced by quartz infrared lamps is less absorbable to PET preforms, more time is required for heating PET preforms before temperature harmonization between the inside and outside of the entire preforms is achieved. Thus, shortcomings resulted by using quartz infrared lamps to heat PET preforms include decreased bottle-blowing efficiency and increased energy consumption.
To solve these problems, many approaches have been proposed recently for improving PET's absorption of infrared rays, with the particularly attempt to make PET preforms absorb near infrared with a wavelength ranging between 1100 and 1200 nm better. Just a few are named below.
U.S. Pat. No. 4,408,004 has taught that the addition of carbon black, as an infrared absorbent material, in a polycondensed resin, reduces the heat-up time required by PET preforms, wherein the carbon black has an average particle size of 10-500 nm and the carbon black is present from 0.1 to 10 ppm by weight of said polyester.
U.S. Pat. No. 5,529,744 disclosed the technique of using gray antimony to improve PET preforms in infrared absorption. The gray antimony as proposed is made by adding a phosphorus (III) reducing agent of during the polycondensation process, so that the trivalent antimony ions, after reacted with a catalyst, can be reduced into the desired gray antimony.
U.S. Pat. No. 6,022,920 uses black iron oxide particles to absorb infrared rays for shortening the time for heating PET preforms, wherein the black iron oxide particle each have a particle size of 0.1-10 μm, and are in the concentration of 5-50 ppm.
U.S. Pat. No. 6,034,167, on the other hand, used graphite as the component for absorbing infrared rays for the same purpose of heating PET preforms fast, wherein the graphite added has a particle size of 0.1-20 μm and a concentration of 0.1-15 ppm.
U.S. Pat. No. 6,503,586 has disclosed the use of inorganic black particles, such as copper chromite spinel, to absorb infrared rays, for reducing the heating time for PET preforms, wherein the inorganic black particles each have a particle size of 0.5-200 μm and are in a concentration of 3-170 ppm.
US Patent Application No, 2006105129 proposes the use of titanium carbide as particles for absorbing infrared, thereby reducing time consumption for heating up PET preforms, wherein the titanium carbide has a particle size of 0.005-100 μm, and a concentration of 0.5-1000 ppm.
WO 2006/055198 also uses titanium carbide to make PET preforms absorb near infrared better.
The above-mentioned prior-art techniques all depend on adding either black or grey inert particles in PET resin to make preforms absorb near infrared better. However, these approaches can unavoidably cause the resultant PET preforms and bottles to become less clear to the extent that they are undesired by beverage manufacturers.